Before you get too excited about being able to breathe on Mars without a spacesuit, think again. Atomic oxygen is very different from the stuff you and I breathe in on a daily basis. However its discovery is important because it helps scientists to determine atmospheric erosion and other gases escaped the planet.
It will also help to determine if those gases might have played a role in supporting life on Mars at some point in time. Scientists currently believe that billions of years ago, Mars lost its magnetic field for some unknown reason, which in turn allowed bursts of energy from the sun to strip it of its atmosphere, leading to the eroded state of the planet that we know today.
The attempt to detect atomic oxygen in the atmosphere has been attempted in the past, like in the 1970s on the Viking and Mariner missions, but back then it was harder as they weren’t able to accurately measure the wavelengths associated with it.